X-ray crystallographic studies on C-phycocyanins from cyanobacteria from different habitats: marine and freshwater.

Abstract

C-phycocyanins from three cyanobacterial cultures of freshwater and marine habitat, Spirulina, Phormidium and Lyngbya spp., were purified to homogeneity and crystallized using the hanging-drop vapour-diffusion method. Blue-coloured crystals in different crystal forms, monoclinic and hexagonal, were obtained for the three species. The crystals took 1-12 weeks to grow to full size using polyethylene glycols of different molecular weights as precipitants. The amino-acid sequences of these proteins show high similarity to other known C-phycocyanins from related organisms; however, the C-phycocyanins reported here showed different biochemical and biophysical properties, i.e. molecular weight, stability etc. The X-ray diffraction data were collected at resolutions of 3.0 A for the monoclinic and 3.2 and 3.6 A for the hexagonal forms. The unit-cell parameters corresponding to the monoclinic space group P2(1) are a = 107.33, b = 115.64, c = 183.26 A, beta = 90.03 degrees for Spirulina sp. C-phycocyanin and are similar for crystals of Phormidium and Lyngbya spp. C-phycocyanins. Crystals belonging to the hexagonal space group P6(3), with unit-cell parameters a = b = 154.97, c = 40.35 A and a = b = 151.96, c = 39.06 A, were also obtained for the C-phycocyanins from Spirulina and Lyngbya spp., respectively. The estimated solvent content is around 50% for the monoclinic crystals of all three species assuming the presence of two hexamers per asymmetric unit. The solvent content is 66.5 and 64.1% for the hexagonal crystals of C-phycocyanin from Spirulina and Lyngbya spp. assuming the presence of one alphabeta monomer per asymmetric unit.